This glossary provides definitions for electron microscopy techniques, terms and acronyms - as well as links to further information on our site. The terms are listed alphabetically, and are quick-linked to their definitions so you can easily find the information you are looking for.

Scanning electron microscope (SEM)

A scanning electron microscope (SEM) produces surface images by scanning a specimen with a beam of high energy electrons in a raster pattern. The electrons interact with the atoms that make up the specimen, producing signals that contain information about the specimen’s surface, topography, composition and other properties.

Depending on the instrument, resolution will vary between less than 1nm and 20nm. In addition to its high resolving capability, the SEM also has a great depth of field, giving the characteristic 3D appearance that is useful for understanding the surface structure of a specimen.

Many SEMs also have the facility to analyse the X-rays given off by the specimen as a result of its electron bombardment and, as each element in the periodic table produces its own X-ray spectrum, this can be used to identify the elemental composition and measure the abundance of elements in the specimen.

SEM cold stage (peltier-cooled)

Low vacuum or variable pressure scanning electron microscopes (SEMs) allow electron microscopists to observe ‘wet’ specimens under low vacuum conditions. However, at room temperature water will quickly evaporate - resulting in considerable changes to the specimen structure. Depending upon the chamber pressure, cooling a wet specimen causes the rate of evaporation to slow or even stop completely. Saturated vapour pressure of water decreases considerably with temperature.

Not only does cooling in low vacuum help prevent changes in specimen structure, but operating at higher vacuum gives a better signal to noise ratio and better image quality.

Further information: K25X Peltier-Cooled SEM Stage

Sputter coater

A sputter coater is a vacuum instrument for depositing thin layers of metals. Its main application in scanning electron microscopy (SEM) is making non-conducting or poorly conducting specimens conductive.

Also see: Sputter coating
Further information: Sputter Coaters and SEM/TEM Carbon Coaters

Sputter coating

Sputter coating in scanning electron microscopy (SEM) is the process of laying down an ultra-thin coating of electrically-conducting metal - such as gold (Au), platinum (Pt), chromium (Cr) or iridium (Ir) - onto a non-conducting or poorly conducting specimen.

Sputter coating prevents charging of the specimen, which would otherwise occur because of the accumulation of static electric fields due to the electron irradiation required during imaging. It also increases the amount of secondary electrons that can be detected from the surface of the specimen in the SEM and therefore increases the signal to noise ratio. Sputtered films for SEM typically have a thickness range of 2-20nm.

Also see: Magnetron sputtering
Further information: Sputter coating technical brief (PDF) on Sputter Coating Techniques and Advantages and Sputter Coaters and SEM/TEM Carbon Coaters

Sputter target

This is the surface being sputtered within a sputter coater. Usually, the sputter target is at a negative electric potential with respect to the chamber plasma. Targets can be formed by machining, rolling, melting, vacuum melting, sintering, CVD and plasma spraying. Sputter targets are consumable items and need to be replaced once the material has been eroded beyond a defined point.

Further information: Sputter Targets

Sublimation

Sublimation is the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase. Sublimation takes place at temperatures and pressures below the triple point of the substance and is endothermic in nature.

Sublimation of water/ice is often a key process in the preparation of water-based specimens for cryo scanning electron microscopy (cryo-SEM). Typically the temperature of the specimen is temporarily raised to a selected point between -80°C and -110°C (depending upon the amount and type of ice to be removed). The sublimation process is arrested by rapidly cooling the specimen back down to below recrystallisation point, typically less than -130°C.

Supercritical drying

An alternative term for critical point drying.

Also see: Critical point drying

TEM sectioning

The production of thin specimen slices. For light microscopy, sections are normally a few micrometers thick, but for electron microscopy (EM) they must be thin enough to be semi-transparent to electrons - typically around 90nm. Ultra-thin sections for transmission electron microscopy (TEM) are cut on an ultramicrotome using glass or diamond knives.

TEM staining

Transmission electron microscopy (TEM) sections are stained with heavy metals, such as lead (Pb) and uranium (U), to scatter imaging electrons and thus give contrast between different structures. This is important as many materials (especially biological tissue) are nearly ‘transparent’ to the electron beam.

Staining the specimens with heavy metals adds electron density, which results in there being more interactions between the electrons in the primary beam and those of the specimen, which in turn provides us with contrast in the resulting image. In biology, specimens can be stained ‘en bloc’ before embedding and/or later, directly after sectioning, by brief exposure of the sections to solutions of the heavy metal stains.

Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) is a technique whereby a beam of electrons is partially transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device - such as a fluorescent screen, on a layer of photographic film or increasingly to be detected by a sensor, such as a CCD camera.

TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small wavelength of electrons, compared to light. This enables the examination of fine specimen detail - even as small as a single column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope. The highest resolution achieved on an aberration-corrected TEM is in the region of 0.5 Ångströms.

The TEM is a major analytical tool in a wide range of scientific fields, in both physical and biological sciences.

Turbomolecular pump

A turbomolecular (turbo) pump is a type of vacuum pump used to obtain and maintain high vacuum. The principle of operation is that gas molecules within a vacuum chamber can be given momentum in a desired direction by repeated collisions with a rapidly spinning turbine rotor. The rotor ‘hits’ gas molecules from the inlet of the pump towards the exhaust in order to create or maintain a vacuum. A turbo pump normally works in tandem with a low-vacuum pump, such as a rotary vacuum pump, which is used to ‘rough pump’ the vacuum system (eg sputter coater or vacuum evaporator) during initial pump-down period, and to ‘back’ the turbo pump (ie remove gases from the back of the pump) during high-vacuum operation.

Ultramicrotome

See: TEM sectioning

Vacuum evaporation

The evaporation of metals or carbon (C) using resistive or electron beam (EB) sources.

Also see: Vacuum evaporator

Vacuum evaporator

An instrument used for thermally evaporating thin films of metals or carbon (C) onto a substrate or specimen. The source material is evaporated in a high vacuum, allowing vapour particles to travel directly to the target object (substrate or specimen), where they condense back to a solid state. The evaporation sources can be electron beam (EB) or thermal, using a resistive heater.

In electron microscopy (EM), vacuum evaporation techniques are now mainly confined to transmission electron microscopy (TEM). Examples include the production of carbon-coated support films on grids and surface replication methods. For scanning electron microscopy (SEM) specimen coating, sputter coating is now generally preferred because the instrumentation has a lower cost and is easier to set up and use. Sputter coating is also a more omni-directional deposition technique, giving more even coating of specimens with irregular surfaces.

Further information: Bench-Top Vacuum Evaporators